1. Field of the Invention
The present invention pertains generally to a method and apparatus for sharpening an image digitally and, in particular, to a robust method for sharpening edges and details of an image digitally without amplifying noise in the input image.
2. Description of the Prior Art
Electronic imaging cameras for recording either motion or still images are well known in the art and in common usage today. Such cameras generally include a two-dimensional photosensitive array which may comprise a high resolution charge coupled device (CCD) or charge injection device (CID) which receives image scene light in a well-known manner by way of an objective lens and shutter. The image sensing array typically comprises a plurality of image sensing elements or pixels arranged in a two-dimensional area array with each image sensing pixel converting the image defining scene light into a corresponding analog voltage value. The image sensing elements are preferably arranged in a plurality of columns and rows and for today's resolution imaging applications may include more than 1,000 columns by 500 rows of image sensing pixels.
When an image is captured by an imaging system, the pixel values are always degraded by random noise from various sources. For example, an image scanned from a print or a film may contain film-grain noise. Actually, it is impossible to conceive of an absolutely noiseless system because of the discrete nature of light. In practical applications, however, the signal-to-noise ratio (SNR) can be made high enough to make such noise relatively invisible to human observers.
It is well known to enhance the image data derived from these light sensing devices to promote both noise reduction and image sharpening. Image sharpening or crispening enhances the sharpness of edges and details of the digital image and, thus, greatly improves the subjective quality of the image. Such enhancement may be accomplished on a pixel-by-pixel basis utilizing the surrounding pixel values of each pixel to be enhanced to provide selectively weighted average pixel values for improved image sharpness and non-weighted average pixel values for noise reduction. Most image enhancing methods provide for both noise reduction and image sharpening; however, noise reduction and image sharpening are inherently contradictory since any reduction in noise provided by nonweighted averaging will reappear upon image sharpening by a selectively weighted average.
A sharpening method which is well known in the art as unsharp masking or Laplacian filtering is based on two-dimensional differentiation and amplifies high-frequency components of image signals. However, it is also well known that sharpening performed in accordance with the unsharp masking method not only enhances edges and details but it also amplifies noise which consists of high-frequency components.
Since sharpening performed in accordance with the unsharp masking method is always achieved at the expense of increasing noise visibility, its use is restricted by the amount of noise which becomes noticeable after the method is applied.
As a result, there is a need in the art for a method and apparatus for sharpening the edges and details of an image without amplifying the noise.